Commercially available sheet finishes



United States Patent ()ffice 3,125,471 COLORED STAINLESS STEELS Jesse R. Conner, Jr., Sarver, Pa., assignor to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., a corporation of Pennsylvania No Drawing. Filed July 3, 1961, Ser. No. 121,353 2 Claims. (Cl. 148-315) This invention relates to improvements in the surface of stainless steels, and is particularly directed to a new article of manufacture that consists of colored stainless steel.

Stainless steels are conventionally provided with highly attractive metallic finishes that are ideal for architectural and decorative applications. Such applications include outdoor and indoor building panels, as well as automobile trim and appliance applications and, consequently, the surface requirements vary broadly and range from the substantially non-reflective finishes commonly employed for outdoor building panel applications, such as No. 1, No. 2D and No. 2B sheet finishes and dull No. 1 strip finishes to the bright finishes usually employed as indoor building panel trim or for automotive and appliance trim such as No. 4, No. 6 and No. 8 sheet finishes or No. 2 automobile strip finish. Other common finishes for architectural applications include special non-reflective building panel finishes provided by grit or shot blasting cold rolling rolls and embossing such finishes on sheet or strip during cold rolling. Fabricators and users of stainless steel have long desired and sought attractive colored stainless steel architectural members that would exhibit the attractive metal finishes such as those enumerated above, but would simultaneously exhibit some color other than the natural silver luster of the metal. A colored stainless steel, particularly one that retains an attractive metallic finish is in great demand. Presently available color coats or paints that may be employed on stainless steel surfaces, with the exception of thin oxide films, are relatively thick pigmented coatings that may be organic or inorganic in nature, but which uniformly hide the desired metallic luster. Also, such paints readily chip and peel in a relatively short period of time when employed for architectural uses. Architectural applications, and particularly outdoor building panel applications, require guarantees that the coloring or coating will not chip or peel for periods exceeding ten years.

Thin oxide coatings obtained by various means through heat tinting of stainless steel, provide interference pattern colors that do not hide or interefere with the natural metallic luster of stainless steel, but add color to the appearance of the stainless steel surface. Such coatings must be extremely thin to avoid taking on the dull, non-metallic, surface-concealing appearance of an oxide scale and, consequently, are insufficiently resistant to slight abrasions and erosive effects to which such coatings must be subjected for either indoor or outdoor architectural applications. Atmospheric conditions readily erode such coatings out of existence within a relatively short period of time, and even for indoor applications, ordinary cleaning with mild abrasives or detergents removes the coating. Consequently, interference pattern oxide coatings are not commercially practical. Attempts to provide resistant-clear coatings over such interference pattern oxide coatings, have been unsuccessful since clear coatings such Patented Mar. 17, 1964 as organic coatings which consist of polymerized organic compounds, themselves do not adequately resist the erosive and corrosive environments of atmospheric conditions or cleaning practices and, additionally, darken in color in a relatively short period of time. Other attempts to provide protective surface coatings for the thin oxide interference color coatings on stainless steel, have resulted in interference with the light reflectivity properties of the oxide coat and have either resulted in altering the color in a non-uniform manner or have caused the stainless steel to take on a multi-colored rainbow-like appearance which changes in accordance with the angle at which one views the so-colored and coated stainless steel article.

It has now been found that a relatively thin, substantially transparent silicate coating on a thin metal oxide intereference colored stainless steel, will so protect the oxide coated article as to render such color coated stainless steel resistant to the effect of atmospheric erosion caused by rain, hail, industrial smoke and fumes, etc., as Well as to provide adequate resistance to the effects of cleaning materials ordinarily employed on any architectural or decorative finish. It has also been found that such silicate coating does not alter the color or the attractive appearance of the surface finish of stainless steel.

Accordingly, it is the object of the present invention to provide a colored stainless steel architectural and decorative member that is resistant to the effects of atmosphere and cleaning.

It is also an object of the present invention to provide a stainless steel surface that is coated with an extremely thin oxide film which provides light interference color patterns to the surface of the stainless steel and a thin silicate coating on the surface of the oxide film which does not alter the coloring effects of the film, but which protects the film from the effect of atmosphere and cleaning.

In general, the present invention is a colored stainless steel article which consists of stainless steel which has been provided with a thin metal oxide film which is capable of providing color by means of light wave interference plus a thin substantially transparent silicate coating covering the oxide coating which does not materially alter the color provided by the oxide coating, but which serves to seal and protect the oxide coating from the erosive and corrosive effects of ordinary atmosphere or cleaning. The invention is particularly directed to architectural stainless steel having on the surface thereof, an intimately bonded metal oxide film having a predetermined thickness within the range of from to 1000 angstroms, and said metal oxide coated stainless steel panel being additionally coated with a substantially transparent, substantially uniform silicate coating no greater in thickness than about .001".

Metal oxide coatings on stainless steel surfaces such as provide the interference pattern colors as employed on the article of the present invention, are commonly obtained by heating cleaned stainless steel in air for a sufficient time for the surface of the stainless steel to acquire the extremely thin oxide coating generally referred to as a heat tint. Such coatings must be extremely thin so that they do not reflect light in the manner of an oxide scale, but instead permit light to penetrate their surfaces and reflect from the surface of the steel itself. Hence, when one observes a lightly heat tinted stainless steel surface, he sees the true metallic finish that the steel exhibited prior to the heat tinting, but also observes an interference color in place of the natural silver appearance of stainless steel. If the thickness of the oxide coating exceeds about 1000 angstroms, the reflected coloring is not attractive and will be inconsistent in its appearance. Thicker coatings do not permit light penetration at all and provide a dull gray or brown appearance commonly associated with an oxide scale. Oxide coatings of less thickness than about 100 angstroms do not provide sufficient observable reflected interference color to be employed in conjunction with the article of the present invention. The metal oxide coatings as presently applied, are crystalline oxide films consisting essentially of solid solutions of Fe O and/ or Fe O plus Cr O In specific instances where the stainless steel contains other alloying ingredients such as molybdenum, copper, nickel, etc., some of the oxides of these metals may also be present; however, it has been found they are never present in such amounts as to alter or interfere with the interference colors as provided by a thickness of oxide coating of 300 to 500 angstroms. The difference between the oxide colors and those produced by most paints or solid coatings is that solid colored coatings effect coloring by the selective absorption of certain wave lengths (intrinsically colored materials), while thin, transparent coatings, such as the oxide coatings, effect coloring by selective interference of light reflected from two different surfaces. The oxide coated stainless steels vary in color from the lighter oxide coatings providing a light yellow or gold to slightly heavier coatings reflecting a bronze appearance, and the heaviest coating reflecting copper, brown, red and blue colors, depending on the exact thickness of the oxide film, which, in turn, depends on the time, temperature and surface conditions wherein the oxide film was created.

The metallic oxide coatings, as provided by the article of the present invention, may be provided most economically and simply to a cleaned stainless steel surface by means of merely heating in air at any temperature of from about 450 F. and up; very slow and faint heat tinting is encountered under about 700 F. and temperatures above about 1800 F. are usually uncontrollable. The best control over uniformity and repeatability of color is obtained when times of greater than two minutes and less than 20 minutes are used. For golds and bronze on most surfaces, temperatures between about 800 F. to 1200 F. are best. Coppers and reds are obtained between about 1000 F. and 1500 F. and the blues and blacks, above about 1500 F. In general, the trend of color with increasing time and temperature is in the following order: pale gold, deep gold, bronze, copper, brown, purple-red, purple, blue and black. Depending upon alloy and surface, some of the colors are missing or are so transient as to be unobtainable in practice. On some surfaces, colors are mottled or exhibit a grain at higher temperatures. Except on very rough surfaces, the colors are all metallic in appearance. When providing the thin oxide coatings which provide interference pattern colors, it is, of course, important that the stainlesss steel surface be clean. If the surface is not clean, the resultant colors are not uniform or repeatable. Any of the known cleaning methods which provide a uniformly clean surface are satisfactory. Useable surfaces may be prepared in some cases by wiping with a drying oil such as linseed oil and rubbing with a clean cloth to remove all excess oil. This gives a surface which while not clean, will usually give uniform color and has little eflect on the color obtained. Alkaline cleaning with, for example, sodium hydroxide is preferred. Although I have found that the metal oxide coatings as applied to the present article, may be most efiiciently acquired through careful, controlled cleaning and heat tinting of stainless steel, such oxide films may be acquired by various other means that are known to industry. For example, such oxide coatings providing interference colors may be produced by heating stainless steel in molten salts or solutions, oil baths, etc., and by various electrolytic treatments. Oxide coatings which fall within the scope of the present invention may be produced by electrolytic treatment in caustic solutions such as is taught in United States Patent No. 2,957,812 to Bongartz et al. The oxide coatings provided by the various methods other than by simple heat tinting are all metal oxide coatings of a similar or identical nature to the heat tinting oxide coatings and all provide interference pattern colors when present as a continuous film, particularly within the range of to 1000 angstroms in thickness and, consequently, all such metal oxide coatings may be employed in the manufacture of the present article. Since the metal oxide coatings, as applied to the article of the present invention, are basically caused by an oxidation of the stainless steel surface itself, it is obvious that such coatings are intricately bonded to the surface of the stainless steel rather than having a mechanical bond as provided by most color coats. The silicate coating provided to protect and seal, but not interfere with the oxide coating, is however, a typical mechanically bonded coating. The silicate coating does not materially alter the appearance of the stainless steel surface which appears to be substantially identical with the non-silicate coated surfaces bearing the same oxide film. In other words, the extremely thin coating presently applied is substantially transparent and can not be ascertained as even being present under ordinary inspection. The silicate coated steel will withstand many hours of corrosive testing, whereas the non-silicate coated steels will lose their col ored appearance within a relatively short period of time.

For the purpose of the present invention, any silicate compound that is soluble in water may be employed to effect the silicate coating. Soluble silicates are, for my purpose, synonymous with alkali metal silicate since only these silicates are soluble in water to any practical extent. The term alkali metal compounds includes such materials as lithium, potassium and sodium; however, as is well known, sodium and potassium are the only such metals that are available in such commercial quantitles as to be considered in any commercially attractive coating process. However, since the invention is operative by employing lithium silicate, such material is included within the scope of the present invention. These soluble-glasses are water soluble solids usually formed by fusion of sand and alkali metal carbonates. The resulting amorphous material may vary widely in its metalsilicon-oxygen ratio; for this reason, the compositions of silicates are usually expressed as SiO :M O ratios (where M 0 is an alkali metal oxide). Examples of some sodium silicates are as follows:

Name Formula SiOzzNmO Ratio Sodium Orthosilieate 1:2 Sodium Metasilicate Na SiO 1:1 Sodium Disilicate Na ShO 2:1 Commercial Wator-Glass", 3.311 Others Approaching Ratio of 4 :1

The ratio of about 4:1 is critical. Above this, the silicate is no longer adequately water soluble, and, in fact, the difiiculty of dissolving the glasses in water increases with the ratio below the critical point so that steam may be used to dissolve those above the disilicate. In solution, these compounds exist in ionic form with the presence of polysilicate ions in the case of the higher ratio silicates such as water glass. Such ions with molecular weights of 200-300 are probably unordered arrangements consisting of both 4 and 6 coordinated silicon atoms in a roughly spherical structure. In a sense, they are semicolloidal in nature with high viscosity.

When such coatings are allowed to evaporate in a thin layer, a substantially transparent film or coating is formed. The thickness of the coating is critical in that if the coating is permitted to exceed about .001" in thickness, it

interferes with the appearance of the colored stainless steel surface and, inasmuch as the attractive colored metallic appearance of the metal itself is its primary asset for decorative trim, it is essential that the thickness of the silicate coating be no greater than about .001". Also, a thicker coating may not be adequately cured in the subsequent baking step and will be more susceptible to efflorescence or bleeding of a white deposit frequently experienced with silicate coatings. Efilorescence is thought to be caused by a rejection of the alkali metal compounds from the coating network.

The baking temperature is not critical because time at temperature is equally important in curing or setting the coating so as to render it substantially insoluble. Coatings which have been dried at room temperature have good mechanical properties but are water soluble. As the baking temperature is increased, the solubility decreases until the coatings are substantially insoluble after baking at 480 F. A fair degree of insolubility is attained at about 300 F., but immersion in water will gradually dissolve such coatings. Times of thirty minutes or less are generally sufiicient. The coating is, of course, baked to render it insoluble. This may be done at temperatures exceeding 300 F. for times of about one minute or more. Satisfactory baking schedules are as follows: 30 minutes at 400 F., 15 minutes at 500 F., minutes at 600 F., 2 minutes at 800 F. The higher temperatures are to be preferred so that the greatest degree of insolubility may be attained.

The ultimate article claimed by applicant, as shown above, has a finish that is glossy and is smooth to the touch depending, of course, on the original surface, and the color has considerable depth. The present invention has been found to be particularly adaptable in providing gold and bronze finishesthe gold finish has considerable depth as compared with gold paints and is much more attractive than these finishes. The coated article is quite resistant to rubbing or scratching type of abrasion, much more so than the base metal or uncoated article. A standard No. 4 pencil and pencil-eraser test demonstration does not scratch the surface of the silicate coated colored article and the coating is resistant to fingerprinting where the finish is such that abrasion resistance is of importance. The coated article is particularly resistant to construction materials such as acids, mortars and calking compounds, as well as food stains, lipstick, etc. Concentrated nitric, sulfuric, hydrochloric and acetic acids, concentrated ammonium and sodium hydroxides and dilute solutions of these reagents have been observed to have little or no apparent effect on the surface of the coated article where the article has been contacted with such materials continuously over a period of up to two hours. Dilute reagents have no effect over a period of at least 24 hours. The resistance of these articles to such chemical attack is particularly significant in view of the fact that prior art unprotected oxidation colors on stainless steel are known to effect a serious loss in corrosion resistance.

In addition, the article of the present invention has been shown to be resistant to all common organic solvents. Solvents actually tested in conjunction with applicants article include benzene, petroleum ether, acetone, xylene, alcohol, carbon tetrachloride, methyl-ethyl ketone and trichloroethylene. These tests are significant in that these are the materials commonly employed as commercial cleaners.

Applicants article in the form of flat rolled stainless steel, such as cold rolled sheet or strip, has shown considerable ability to withstand bending, forming and other mechanical deformations without serious alteration of the color or coating. In general, the thicker the silicate coating within the range specified and the smoother the surface, the less flexibility. Coatings of up to .001 maximum can usually be bent 90 around a sharp radius without harm. The larger the radius the greater the possible bend. With .037 gauge type 302 stainless steel with a 2 DVB sheet finish and having a gold heat tint and a thin (less than .001) silicate coating, the following tests did not harm the finish: sharp bend, bend around A inch mandrel, 80 inchpound surface or reverse-side impact test, shearing, stretcher-leveling and twisting into a spiral.

The solution employed to effect the silicate coating may be sprayed upon the surface of the oxide colored stainless steel or the interference colored stainless steel may be dipped into such a solution so as to effect the desired films. Additionally, the silicate coatings may be applied to strip or sheet by being rolled on. Such process is a commercially known coating means wherein the coating material is constantly applied to the surface of a roll which is, in turn, rolled over the surface of the metal strip or sheet.

The concentration of alkali metal silicate present in the aqueous solution is not critical in that any concentration may be employed that will effect such a film, and nearly any amount from a trace to saturation will effect a film which varies in thickness in accordance with the strength of the solution employed. For practical purposes, it would be necessary to employ a solution containing at least about .1%, by weight, of the alkali metal silicate to effect a .001 thick coating. It is entirely feasible, however, to employ aqueous solutions that are saturated with the silicate compound. The temperature of application is also unimportant so long as at least about .l% of the silicate is present in the water solution.

In applying the silicate coating it is preferable, though not essential, to add to the vehicle or coating solution a small quantity of a wetting agent. Such wetting agent may be any of the commercially available water soluble wetting agents that are compatible with the silicate solutions. The effect of such additions is to aid in securing a smooth and even coating on the surface being silicate coated. Excellent results have been obtained by employing 10' ml. of a 10%, by weight, Aerosol AY solution with a 75 ml. sodium silicate solution (technical, 4042 B.). Aerosol AY is the trade name of the American Cyanamid Company, 30 Rockefeller Plaza, New York 20, New York, and is designated to identify solutions containing as the active ingredient diamyl sodium sulfosuccinate. Another commercially available wetting agent that has been satisfactorily employed is a modified sodium lauryl sulfate sold under the trade name of Wetanol by the Glyco Products Company, Empire State Building, New York, New York.

The term stainless steel, as applied in the present specification, is intended to include all the steels classified by the American Iron and Steel Institute as being standard grades of stainless steel. These include the type 400 series stainless steels that contain chromium in amounts of from about 10%, by weight, to about 30%, by weight, and generally less than 1% carbon, such as AISI types 410 and 430 and additionally the A181 type 300 series which contain, in addition to Cr and C, a nickel content of from 6 to 30% which renders the steel structure austenitic, such as AISI type 301, 302 and 304, and the 200 series steels which contain not only nickel in amounts of 1 to 10%, but also up to about 30% Mn and .60% N as additional austenitizers. Such various stainless steel analyses may contain additionally, as impurities or alloying ingredients, small amounts of P, S, Cu, Mo, Se, B, Be, Co, W, Ti, Cb, Ta, V, Zn, Al, Si, rare earths, etc. All stainless steels, however, contain chromium within the range of from about 10% to 30% and carbon up to about 1%. The Cr content in every instance is the element that primarily effects the essential property of oxidation and corrosion resistance, and consequently the article of the present invention may be broadly said to be composed partly of a steel that consists essentially of carbon COMMERCIALLY AVAILABLE SHEET FINISHES No. 1 Hot rolled, annealed and pickled. No. 2D Full finish, dull cold rolled. No. 2B Full finish, bright cold rolled.

Any of the above standard finishes may be varied. For example, a No. 2 DVB finish is used to identify an architectural finish which is provided by grit or shot blasting cold rolling rolls and embossing such finishes on sheet or strip during cold rolling. The latter finish and the means for obtaining it are fully disclosed in co-pending United States patent application Serial No. 67,889.

Applicants article is readily employed as outdoor or indoor architectural building panels or members, but can also be employed for other decorative uses such as automobile trim or decorative members for appliances. It is therefore understood that for purposes of the present specification and claims, the term architectural member shall include such similar articles as decorative trim and other decorative members such as door and window trim and hardware and in domestic appliances and the like.

The following specific examples are given to illustrate the best mode of preparation of the articles of the present invention. All have been corrosion tested in the manner described above and have been found to be satisfactory from the standpoint of appearance, wear and abrasion resistance, resistance to soiling, chemical resistance, solvent resistance, resistance to corrosion testing and have shown adequate flexibility:

Example 1 AISI type 302 stainless steel panels with a No. 4 sheet finish were cleaned by scrubbing with an alkaline cleaner and rinsing in clear water. They were then heated in air in electric mufile furnace for 10 minutes at 1000" F. to produce a uniform gold color. It was then cooled, sprayed with 30% sodium silicate solution (30% 40-42 B. sodium silicate solution+.1% Wetanol in water) and baked at 600 F. for 5 minutes.

Example 2 AISI type 302 stainless steel panels with a 2 DVB finish were cleaned by scrubbing with an alkaline cleaner and rinsing in clear water. They were then heated in air for minutes at 1000 F. to produce a uniform bronze color. They were then roller-coated with a sodium silicate solution and baked at 800 F. for 2 minutes.

Example 3 AISI type 201 stainless steel panels wtih a bright annealed finish were cleaned by scrubbing with an alkaline cleaner and rinsing in clear water. They were then heated in air for 12 minutes at 1000 F. to produce a uniform copper color. They were then dip-coated in 50% sodium silicate solution and baked at 400 F. for 30 minutes.

Example 4 AISI type 430 stainless steel panels with a No. 2 strip finish were scrubbed with an alkaline cleaner and rinsed in clear water. They were then heated in air for 2 minutes at 1200 F. and some panels were heated for 15 minutes at 1450 F., both treatments of which resulted in uniform metallic purple or metallic blue finishes respectively. All panels were flow-coated with a sodium silicate solution and baked at 500 F. for 10 minutes.

The surfaces of stainless steel panels, such as were prepared in Examples 1 through 4, were found to be resistant to scratching and abrasion when rubbed with the eraser of a standard No. 4 pencil. Drops of concentrated nitric, sulfuric, hydrochloric and acetic acids, concentrated ammonium and sodium hydroxides and dilute solutions of each of these reagents, were placed on the surface of these panels. There was little or no apparent effect on the panel surfaces after standing for a period of about two hours at room temperature. Dilute reagents (about 10% by volume of the concentrated chemicals in water, except NaOH which was about 10%, by weight) had no effect over a period of 24 hours. No adverse effects were observed by immersing the samples in benzene, petroleum ether, acetone, xylene, alcohol, carbon tetrachloride, methyl-ethyl ketone and trichloroethylene.

I claim:

1. A stainless steel architectural member comprising eat tinted stainless steel having a visible metal oxide coating thereof produced by said heat tinting of from about to 1000 A. in thickness integrally bonded to the surface thereof and a substantially transparent and water-insoluble baked alkali metal silicate coating on said metal oxide coating, said silicate coating not exceeding about .001 in thickness.

2. A stainless steel architectural member comprising heat tinted fiat rolled stainless steel having a visible metal oxide coating thereof produced by said heat tinting of from about 100 to 1000 A. in thickness integrally bonded to the surface thereof and a substantially transparent and water-insoluble baked alkali metal silicate coating on said metal oxide coating, said silicate coating not exceeding about .001" in thickness.

References Cited in the file of this patent UNITED STATES PATENTS 1,428,170 Laist et al. Sept. 5, 1922 2,069,486 Tilden Feb. 2, 1937 2,440,969 Nightingall May 4, 1948 2,492,682 Carpenter Dec. 27, 1949 2,521,580 Hornak et al. Sept. 5, 1950 2,952,562 Morris et al. Sept. 13, 1960 3,016,339 Riou Jan. 9, 1962 

1. A STAINLESS STEEL ARCHITECTURAL MEMBER COMPRISING HEAT TINTED STAINLESS STEEL HAVING A VISIBLE METAL OXIDE COATING THEREOF PRODUCED BY SAID HEAT TINTING OF FROM ABOUT 100 TO 1000 A. IN THICKNESS INTEGRALLY BONDED TO THE SURFACE THEREOF AND A SUBSTANTIALLY TRANSPARENT AND WATER-INSOLUBLE BAKED ALKALI METAL SILICATE COATIANG ON SAID METAL OXIDE COATING, SAID SILICATE COATING NOT EXCEEDING ABOUT .001" IN THICKNESS. 